The present invention relates to a liquid crystal display device in which it is possible to avoid corrosion of the signal lines thereof
A liquid crystal display device includes a panel made of substrates disposed so as to be opposed to each other with a liquid crystal interposed therebetween, and the display device is provided with numerous pixels disposed in a matrix arrangement over the area of the liquid crystal. In the case of a reflection type of liquid crystal display device, at least one of the pair of substrates is a transparent substrate; whereas in the case of a transmission type of liquid crystal display device, both substrates are transparent substrates. The term xe2x80x9ctransparent substratexe2x80x9d used herein is not limited to a substrate in which tight absorption does not at all occur, but encompasses a substrate which allows light propagating through the liquid crystal to be transmitted through the substrate at an intensity sufficient to enable a user of the liquid crystal display device to recognize an image reproduced by the propagating light. A transmission type liquid crystal display device will be described below by way of example.
Means for generating an electric field which corresponds to a video signal is incorporated into each pixel arranged on the liquid-crystal-side (or the liquid-crystal-layer-side) of the main surface of one of the pair of transparent substrates, whereby the optical transmissivity of the liquid crystal can be controlled according to the strength of the electric field. For this reason, signal lines which supply signals to the respective pixel areas are formed over the liquid-crystal-side main surface of the one of the pair of substrates, and these signal lines are disposed to extend outward of the panel.
Specifically, the one transparent substrate (the substrate over which the signal lines are formed) of the pair of transparent substrates constituting the panel is constructed to be slightly larger in area than the other transparent substrate, and the signal lines are disposed so as to extend into the area of the one transparent substrate which extends beyond the other substrate (in other words, the area of the one transparent substrate which protrudes beyond and is not overlapped by the other transparent substrate, and these signal lines are connected to an external circuit.
Incidentally, a sealing material is arranged in a portion equivalent to the periphery of the other transparent substrate, whereby the other substrate is fixed to the one transparent substrate. These transparent substrates are combined so that a predetermined gap is maintained between the main surfaces thereof, and the liquid crystal is sealed in the gap. Concretely, a region surrounded by the sealing material is formed in a space lying between the pair of substrates, and this region forms a space for containing the liquid crystals.
However, in, the liquid crystal display device constructed in the manner described above, it has been formed that disconnections due to corrosion occasionally will occur in the signal lines which extend from the inside of the region surrounded by the sealing material (the area in which the liquid crystal is sealed) to the outside thereof A technique for solving the problem of corrosion of the signal lines is described in, for example, each of Japanese Patent Application Laid-Open Nos. 326942/1999, 113922/1997 and 90394/1997.
The signal lines which extend from the inside of the area surrounded by the sealing material have heretofore been covered with a film formed by expanding a protective film, disposed in a display area comprising an assembly of pixels, to the edge portions of the substrates (hereinafter referred to as the expanded film). The present inventors have carried out research to determine why the above-described corrosion of the signal lines occurs even in a liquid crystal display device having this construction, and have found that, if there is a defect, such as a pinhole in the expanded film, a signal line disposed below the defective portion tends to corrode.
In recent years, the distance between a signal line and another signal line adjacent thereto is becoming smaller and the electric field strength therebetween is becoming larger, so that there is a stronger possibility that the above-described corrosion of the signal lines will be promoted. In addition, even if no disconnection occurs, there is no denying the fact that the corrosion of a signal line may reach the bottom of a sealing material, so that a leakage hole may be produced in the sealing material and a leakage defect of the liquid crystal may occur.
The present invention has been made on the basis of the above-described circumstances, and an object of the present invention is to provide a liquid crystal display device which is capable of preventing corrosion from occurring in signal lines which are formed to extend from inside an area of a main surface of one of a pair of substrates into an outside area, the inside area being generally surrounded by a sealing material (an area opposed to a liquid crystal layer).
A representative aspect of the invention disclosed in the present application will be described below in brief
The liquid crystal display device of the present invention has a pair of substrates disposed to oppose one another with a liquid crystal interposed therebetween; a plurality of pixel areas formed over a liquid-crystal-side surface of one of this pair of substrates; and a plurality of signal lines for respectively supplying signals to the aforementioned plurality of pixel areas being formed over the liquid-crystal-side surface of the one of this pair of substrates, which signal lines extend beyond a seating material for fixing this pair of substrates to one another and toward a peripheral portion of the one of this pair of substrates; and at least one of the plurality of signal lines is covered with stacked layers formed by stacking a first insulating film, a semiconductor layer, and a second insulating film thereover in this order.
In this manner, the signal lines which are formed to extend beyond the sealing material are covered with the stacked layer made of the first insulating film, the semiconductor layer and the second insulating film. Accordingly, even if defects, such as pinholes, occur in each of the first insulating film, the semiconductor layer and the second insulating film, it will be extremely rare for the portions of the defects to be superposed on one another on the signal lines, and so almost no external influence is exerted over the signal lines due to these defects. In this way, it is possible to avoid corrosion due to defects, such as pinholes, in the material which covers the signal lines.
In the construction described above, each of the aforementioned plurality of pixel areas is provided with a thin film transistor covered with a protective film, and the first insulating film, the semiconductor layer and the second insulating film are formed of materials which are identical to those of a gate insulating film and a semiconductor layer, both of which constitute elements of the thin film transistor, and the protective film covering the, thin film transistor, respectively.
In this case, since the first insulating film, the semiconductor layer and the second insulating film can be sequentially formed in parallel during formation of each of the pixels, there is the advantage that the present invention can be implemented without any increase in the number of manufacturing steps.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.